Testing for mould in your house is beating me

Probably because the DIY petri dish kits are close to useless. Mould spores are in every house, so the dish grows something whatever you do, and it can't tell you where it's coming from. Your nose and a cheap hygrometer are better tools. Musty smell, check behind furniture against outside walls, under sinks, around window frames, in the loft. If humidity sits above about 60 percent most of the time, that's your real problem. If you suspect something hidden in a wall, that's when a proper inspector with air samples compared against an outdoor reading is worth paying for.